In recent years, since semiconductor lasers are small and have low power consumption, they have been used for many information devices. Among them, there is an external cavity type semiconductor laser that emits a beam whose wavelength is stabilized by an external beam having a predetermined wavelength.
Next, with reference to FIG. 1, a typical Littrow type semiconductor laser will be described. A multiple longitudinal mode laser beam emitted from a semiconductor laser device, such as a laser diode 100, is collimated by a lens (collimate lens) 101. The collimated beam reaches a grating 102. The grating 102 outputs a beam having a predetermined wavelength as a first order diffracted beam 103 corresponding to an arrangement angle of the grating 102. The first order diffracted beam 103 is reversely injected into the laser diode 100 through the lens 101. As a result, the laser diode 100 resonates with the injected first order diffracted beam and emits a single mode beam. The wavelength of the beam emitted from the laser diode 100 is the same as the wavelength of the first order diffracted beam 103 outputted from the grating 102. The rest of the laser beam that has reached the grating 102 is a 0-th order beam 104. The 0-th order beam 104 is reflected at the same angle as the incident angle.
Next, with reference to FIG. 2 and FIG. 3, a structure of a laser system that has a typical external cavity type semiconductor laser that is commercially available will be described. FIG. 2 is a plan view showing a laser system 120. FIG. 3 is a front view showing the laser system 120 seen toward a direction denoted by C in FIG. 2. The structure of the laser system 120 is the same as that of a laser system described in a periodical, L. Ricci, et al. “A compact grating-stabilized diode laser system for atomic physics”, Optics Communications, 117 1995, pp 541-549.
The laser system 120 shown in FIG. 2 and FIG. 3 is composed of a laser section 130 that has a laser diode 121, a lens (collimate lens) 122, a grating 123, a first supporting portion 124, a first screw 125, a first groove 126, a second supporting portion 127, a second screw 128, and a second groove 129; a Peltier device 141; and a temperature control section 143 that has a heat sink 142.
As are clear from FIG. 2 and FIG. 3, optical parts such as the lens 122 and the grating 123 are horizontally arranged on a mounting surface of the laser system 120. An optical path of a laser beam is nearly in parallel with the mounting surface. The temperature control section 143 is disposed below the laser section 130. The temperature control section 143 controls temperatures of structural components of the laser section 130 such as the laser diode 121 and the lens 122. The temperature control section 143 keeps the temperature of the laser diode 121 constant. As a result, a light source, which is the laser diode 121, is stabilized.
As shown in FIG. 1, when the arrangement angle of the grating 123 of the laser system 120 is changed, a wavelength of the first order diffracted beam that travels from the grating 123 to the laser diode 121 varies. As a result, a wavelength of an oscillated beam of the laser diode 121 is adjusted. A 0-th order beam D reflected on the grating 123 is emitted to the outside.
The grating 123 is held by the first supporting portion 124. The first supporting portion 124 has the first groove 126. When the first screw 125 mounted in the first supporting portion 124 is rotated, a gap of the first groove 126 is partly widened or narrowed. As a result, a horizontal arrangement angle of the grating 123 slightly varies.
The similar mechanism that adjusts a vertical angle of the grating 123 is disposed. The first supporting portion 124 that supports the grating 123 is supported by the second supporting portion 127. The second supporting portion 127 has the second groove 129. Likewise, when the second screw 128 mounted in the second supporting portion 127 is rotated, a gap of the second groove 129 is partly widened or narrowed. As a result, a vertical arrangement angle of the first supporting portion 124 and the grating 123 slightly varies.
Blue laser diodes as laser diodes 121 have been developed by several makers. These makers have begun using these laser diodes for optical discs and so forth. In addition, as described above, an external cavity type laser system has been used for a holography memory writer that needs a single mode laser beam.
The maximum output of a commercially available laser system of the same type is up to 15 mW as a catalog value.
However, when the external cavity type semiconductor laser is used for a holography memory writer and so forth, a laser output of 30 mW or higher is needed. Conventional external cavity type semiconductor lasers have not yet accomplished such an output level.
The external cavity type semiconductor lasers accomplish a single mode with an external cavity. However, they may not accomplish a sufficient single mode characteristic due to various factors.
Therefore, an object of the present invention is to provide an external cavity type semiconductor laser that has the two features of a larger output and a more excellent single mode characteristic than conventional external cavity type semiconductor lasers or an external cavity type semiconductor laser having one of these features.
Another object of the present invention is to provide an external cavity type semiconductor laser that has a laser beam output of 30 mW or higher and that is capable of being used for a holography memory writer and so forth.